Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donor and recipient and by use of immunosuppressant drugs after transplant.[1]

The first successful organ transplant, performed in 1954 by Joseph Murray, involved identical twins, and so no rejection was observed. Otherwise, the number of mismatched gene variants, namely alleles, encoding cell surface molecules called major histocompatibility complex (MHC), classes I and II, correlate with the rapidity and severity of transplant rejection. In humans MHC is also called human leukocyte antigen (HLA).

Though cytotoxic-crossmatch assay can predict rejection mediated by cellular immunity, genetic-expression tests specific to the organ type to be transplanted, for instance AlloMap Molecular Expression Testing, have a high negative predictive value. Transplanting only ABO-compatible grafts (matching blood groups between donor and recipient) helps prevent rejection mediated by humoral immunity.

Because very young children (generally under 12 months, but often as old as 24 months[2]) do not have a well-developed immune system,[3] it is possible for them to receive organs from otherwise incompatible donors. This is known as ABO-incompatible (ABOi) transplantation. Graft survival and patient mortality is approximately the same between ABOi and ABO-compatible (ABOc) recipients.[4] While focus has been on infant heart transplants, the principles generally apply to other forms of solid organ transplantation.[2]

The most important factors are that the recipient have not produced isohemagglutinins, and that they have low levels of T cell-independent antigens.[3][5] UNOS regulations allow for ABOi transplantation in children under two years of age if isohemagglutinin titers are 1:4 or below,[6][7] and if there is no matching ABOc recipient.[6][7][8] Studies have shown that the period under which a recipient may undergo ABOi transplantation may be prolonged by exposure to nonself A and B antigens.[9] Furthermore, should the recipient (for example, type B-positive with a type AB-positive graft) require eventual retransplantation, the recipient may receive a new organ of either blood type.[2][7]

Limited success has been achieved in ABO-incompatible heart transplants in adults,[10] though this requires that the adult recipients have low levels of anti-A or anti-B antibodies.[10] Kidney transplantation is more successful, with similar long-term graft survival rates to ABOc transplants.[7]

An animal's exposure to the antigens of a different member of the same or similar species is allostimulation, and the tissue is allogenic. Transplanted organs are often acquired from a cadaver (usually a host who had succumbed to trauma), whose tissues had already sustained ischemia or inflammation.

When memory helper T cells' CD4 receptors bind to the MHC class II molecules which are expressed on the surfaces of the target cells of the graft tissue, the memory helper T cells' T cell receptors (TCRs) can recognize their target antigen that is presented by the MHC class II molecules. The memory helper T cell subsequently produces clones that, as effector cells, secrete immune signalling molecules (cytokines) in approximately the cytokine balance that had prevailed at the memory helper T cell's priming to memorize the antigen. As the priming event in this instance occurred amid inflammation, the immune memory is pro-inflammatory.

As a cell is indicated by the prefix cyto, a cytotoxic influence destroys the cell. Alloreactive killer T cells, also called cytotoxic T lymphocytes (CTLs), have CD8 receptors that dock to the transplanted tissue's MHC class I molecules, which display the donor's self peptides. (In the living donor, such presentation of self antigens helped maintain self tolerance.) Thereupon, the T cell receptors (TCRs) of the killer T cells recognize their matching epitope, and trigger the target cell's programmed cell death by apoptosis.

Developed through an earlier primary exposure that primed specific immunity to the nonself antigen, a transplant recipient can have specific antibody crossreacting with the donor tissue upon the transplant event, a secondary exposure. This is typical of minor blood group exposure (e.g. Kell) following allogenic blood transfusion or trauma during pregnancy. At secondary exposure, these crossreactive antibody molecules interact with aspects of innate immunity—soluble immune proteins called complement and innate immune cells called phagocytes—which inflames and destroys the transplanted tissue.

Secreted by an activated B cell, then called plasma cell, an antibody molecule is a soluble immunoglobulin (Ig) whose basic unit is shaped like the letter Y: the two arms are the Fab regions, while the single stalk is the Fc region. Each of the two tips of Fab region is the paratope, which binds a matching molecular sequence and its 3D shape (conformation), altogether called epitope, within the target antigen.

The IgG's Fc region also enables opsonization by a phagocyte, a process by which the Fc receptor on the phagocyte—such as neutrophils in blood and macrophages in tissues—binds the antibody molecule's FC stalk, and the phagocyte exhibits enhanced uptake of the antigen, attached to the antibody molecule's Fab region.

When the paratope of Ig class gamma (IgG) binds its matching epitope, IgG's Fc region conformationally shifts and can host a complement protein, initiating the complement cascade that terminates by punching a hole in a cell membrane. With many holes so punched, fluid rushes into the cell and ruptures it.

While kidneys can routinely be obtained from human donors, most organs are in short supply leading to consideration of xenotransplants from other species. Pigs are especially likely sources for xenotransplants, chosen for the anatomical and physiological characteristics they share with humans.[11] However, the sugar galactose-alpha-1,3-galactose (αGal) has been implicated as a major factor in hyperacute rejection in xenotransplantation. Unlike virtually all other mammals, humans and other primates do not make αGal, and in fact recognize it as an antigen.[12] During transplantation, xenoreactive natural antibodies recognize αGal on the graft endothelium as an antigen, and the resulting complement-mediated immune response leads to a rejection of the transplant.[13]

Developing with formation of cellular immunity, acute rejection occurs to some degree in all transplants, except between identical twins, unless immunosuppression is achieved (usually through drugs). Acute rejection begins as early as one week after transplant, the risk being highest in the first three months, though it can occur months to years later. Highly vascular tissues such as kidney or liver often host the earliest signs—particularly at endothelial cells lining blood vessels—though it eventually occurs in roughly 10 to 30% of liver transplants, and 10 to 20% of kidney transplants. A single episode of acute rejection can be recognized and promptly treated, usually preventing organ failure, but recurrent episodes lead to chronic rejection. It is believed that the process of acute rejection is mediated by the cell mediated pathway, specifically by mononuclear macrophages and T-lymphocytes.

The term chronic rejection initially described long-term loss of function in transplanted organs via fibrosis of the transplanted tissue's blood vessels. This is now chronic allograft vasculopathy, however, leaving chronic rejection referring to rejection due to more patent aspects of immunity.

Chronic rejection explains long-term morbidity in most lung-transplant recipients,[14][15] the median survival roughly 4.7 years, about half the span versus other major organ transplants.[16] In histopathology the condition is bronchiolitis obliterans, which clinically presents as progressive airflow obstruction, often involving dyspnea and coughing, and the patient eventually succumbs to pulmonary insufficiency or secondary acute infection.

Airflow obstruction not ascribable to other cause is labeled bronchiolitis obliteranssyndrome (BOS), confirmed by a persistent drop—three or more weeks—in forced expiratory volume (FEV1) by at least 20%.[17] BOS is seen in over 50% of lung-transplant recipients by 5 years, and in over 80% by ten years. First noted is infiltration by lymphocytes, followed by epithelial cell injury, then inflammatory lesions and recruitment of fibroblasts and myofibroblasts, which proliferate and secrete proteins forming scar tissue.[18] Generally thought unpredictable, BOS progression varies widely: lung function may suddenly fall but stabilize for years, or rapidly progress to death within a few months. Risk factors include prior acute rejection episodes, gastroesophageal reflux disease, acute infections, particular age groups, HLA mis-matching, lymphocytic bronchiolitis, and graft dysfunction (e.g., airway ischemia).[19]

One principal reason for transplant rejection is non-adherence to prescribed immunosuppressant regimens. This is particularly the case with adolescent recipients,[20] with non-adherence rates near 50% in some instances.[20]

Hyperacute rejection manifests severely and within minutes, and so treatment is immediate: removal of the tissue. Chronic rejection is generally considered irreversible and poorly amenable to treatment—only retransplant generally indicated if feasible—though inhaled ciclosporin is being investigated to delay or prevent chronic rejection of lung transplants. Acute rejection is treated with one or several of a few strategies. Despite treatment, rejection remains a major cause of transplant failure [26]

Gene therapy is another method that can be used. In this method, the genes that cause the body to reject transplants would be deactivated. Research is still being conducted, and no gene therapies are being used to date to treat patients.[27][28][29][30] Current research tends to focus on Th1 and Th17 which mediate allograft rejection via the CD4 and CD8 T cells[31]